1. Field of the Invention
The present invention relates to gas pressure sensors and, more particularly, to a gas pressure sensor that has resistance to electromagnetic waves, applied from an external source, that is, a gas pressure sensor that is excellent in an electro magnetic compatibility (EMS: Electro Magnetic Compatibility).
2. Description of the Related Art
In the past, digital equipment has heretofore been widely used. With digital technology utilized in general, digital equipment creates electro magnetic waves, serving as interfering waves, in a wide frequency range, causing issues of various disturbances to other digital equipment.
In particular, digital equipment is adversely affected with the electro magnetic waves and radio waves released from other digital equipment to suffer defects such as degradation in function, erroneous operation and unintended halts. This is because the electro magnetic waves and radio waves, taken in digital equipment from external sources, make noise which results in failures in a circuit of digital equipment. With the occurrence of such issues, research and development work has been undertaken to provide digital equipment in practical use with improved resistance to noises so as to minimize adverse affects to be caused on digital equipment.
A gas pressure sensor, adapted for installation on a vehicle, serves as digital equipment that is susceptible to adverse affects caused by the electro magnetic waves. In particular, the electro magnetic waves, generated from an ECU mounted on the vehicle, transfer via wirings to which the gas pressure sensor is connected and are taken as noises in a circuit inside the gas pressure sensor, causing problems to occur in the gas pressure sensor.
In recent years, attempts have been made to devise a gas pressure sensor that improves resistance to noises resulting from electro magnetic waves, that is, a gas pressure sensor with improved noise resistance.
FIG. 6 is a schematic cross-sectional view showing a gas pressure sensor of the related art. Hereinafter, a structure of the related art pressure sensor is described below with reference to FIG. 6.
The gas pressure sensor, shown in FIG. 6, is comprised of a housing J1, a stem J2, a threaded member J3, a substrate J4, a pin component J5, a feedthrough capacitor J6, a terminal component J7 and a connector J8.
Mounted inside the housing J1, which plays a role as a body earth, is the hollow stem J2 that includes a cylindrical hollow shaft whose one end is formed with a thin-walled diaphragm J21 serving as a closed portion and the other end formed with a channel. A sensor chip (sensor) J22 is fixedly secured to the diaphragm J21 of the stem J21, which is fixedly secured to the housing J1 by means of the threaded member J3 so as to allow the channel J23 of the stem J22 to communicate with a gas pressure passage (gas pressure passage) of the housing J1. This allows the other end of the stem J2 to be pressed against an opening peripheral edge of the gas pressure passage J11 to be kept in a sealing effect.
Placed on the threaded member J3 is the substrate J4 that is applied with a signal detected by the sensor chip J22 mounted on the diaphragm J21. The substrate J4 carries thereon an IC chip by which an output of the sensor chip J22 is amplified, an IC chip by which the output of the gas pressure sensor is regulated, a signal processing circuit, and an associated wiring pattern. The sensor chip J22 and the circuit on the substrate J4 are bonded to each other by wires J41.
Further, a pin J42 is fixedly secured to the substrate J4 by silver brazing to output the signal to the outside, and the pin J42 is joined to the pin component J5, mounted on the substrate J4, by laser welding. Disposed over the pin component J5 is a feedthrough capacitor J6 that is composed of a ceramic capacitor J61.
Furthermore, disposed over the feedthrough capacitor J6 is a terminal component J7 that includes terminals (for supplying electric power, grounding and outputting an output signal) J71, and the pin J42, mounted on the substrate J4, is electrically connected to the terminal through the pin component J5 and the feedthrough capacitor J6. A connector case J8 is tightly fitted to the housing J1 from an upper area of the terminal component J7 and caulked at an upper end of the housing J1 to be fixedly secured thereon, thereby forming the gas pressure sensor.
Among the components of the gas pressure sensor set forth above, the feedthrough capacitor J6 serves to minimize adverse affects caused by the electromagnetic waves applied to the gas pressure sensor from the external sources. The feedthrough capacitor J6 is comprised of a ceramic capacitor J61, which includes a ceramic layer sandwiched between metallic plates, and a metallic component J62 by which the ceramic capacitor J61 is fixedly secured. With the ceramic capacitor J61, one electrode is electrically connected to the respective terminal J71. The other electrode of the ceramic capacitor J61 is joined to a metallic plate component J62 whose distal end J63 is held in close contact with the housing J1 such that the ceramic capacitor J61 is grounded to the housing J1.
Such a gas pressure sensor with the structure set forth above is configured such that if the noises are inputted from the respective terminal J71, the noises are outputted to the housing J1 via the feedthrough capacitor J6. In such a way, the circuit of the substrate inside the gas pressure sensor is enabled to eliminate the adverse affects resulting the noises, thereby enhancing noise resistance in the gas pressure sensor. Accordingly, the gas pressure sensor is able to function without causing any trouble due to the noises inputted through the wirings to which the gas pressure sensor is connected.
However, in order to have noise resistance, a need arises for the gas pressure sensor of the above related art to be provided with the feedthrough capacitor J6. This results in increases in the number of component parts, which constitute the gas pressure sensor, and an assembling process increases in steps. Particularly, in order for the feedthrough capacitor J6 to be mounted, there is a need for the pin component J5 and the terminal component J7 to be additionally provided. The increase in the number of component parts of the gas pressure sensor leads to an increase in manufacturing costs of the gas pressure sensor.
A major reason why the gas pressure sensor includes a large number of component parts comes from the fact that the feedthrough capacitor J6 is provided in the gas pressure sensor. However, the feedthrough capacitor J6 serves to preclude the occurrence of defects in the gas pressure sensor due to the adverse affects resulting from the electromagnetic waves under circumstances where the gas pressure sensor is placed in a location susceptible to the electro magnetic waves from the external source. Thus, the feedthrough capacitor J6 is indispensable for the gas pressure sensor to be normally operative. Therefore, it is conceived that if the gas pressure sensor has no feedthrough capacitor J6, remarkable degradation occurs in noise resistance of the gas pressure sensor.
To address such an issue, an attempt may be conceivably made to replace the feedthrough capacitor, with a large size in structure, with a chip capacitor, with a smaller size in structure than the feedthrough capacitor, to allow the chip capacitor to be mounted on the substrate J4. With such an attempt, it is conceived that no need arises for using the pin component J5 and the terminal component J7, through which the terminal J71 and the circuit of the substrate J4 are electrically connected, and the gas pressure sensor is able to reduce the number of component parts,
However, the chip capacitor takes the form of a structure composed of stacks of many ceramic and electrode layers and a probability occurs wherein a high frequency current is hard to flow due to the occurrence of hypothetical resistance components among the electrodes. That is, there is a probability for the chip capacitor to have a difficulty of obtaining a frequency characteristic equivalent to that of the feedthrough capacitor J6 through which a high frequency current is enabled to flow.
If the chip capacitor encounters the difficulty of obtaining the frequency characteristic equivalent to that of the feedthrough capacitor J6, then, the chip capacitor has a narrow frequency range for a signal to be able to pass through the chip capacitor. That is, for the frequency characteristic of the chip capacitor, it is conceived that if a noise, falling in a frequency outside the frequency range, is inputted to the gas pressure sensor, the noise is not inputted to the chip capacitor but is directly inputted to the circuit of the substrate J4, causing problems to occur in the circuit of the substrate J4.
Further, when using the chip capacitor, one electrode of the chip capacitor is electrically connected to the terminal J71 and internal circuit of the substrate J4 while the other electrode is electrically connected through the metallic plate component J62 to the housing J1 as described above. However, the noises, inputted from the terminal J71, flow into a branch point of the wiring between the circuit of the substrate J4 and the chip capacitor, and all of the noises inputted from the terminal J71 do not always flow into the chip capacitor. If the noises do not flow to the chip capacitor, the noises flow to the circuit of the substrate J4, causing problems to occur in the circuit of the substrate J4.
Therefore, even if the feedthrough capacitor J6 is replaced with the chip capacitor, there is a probability for the gas pressure sensor, employing the chip capacitor, to encounter a difficulty in obtaining noise resistance equivalent to that of the feedthrough capacitor J6 used in the related art.